213 FXUS66 KPQR 270955 AFDPQR Area Forecast Discussion National Weather Service Portland OR 255 AM PDT Mon Mar 27 2017 .SYNOPSIS...An upper level trough will maintain showers today, decreasing during the afternoon and evening as high pressure builds into the region. This will bring a brief break in the rain for most of the forecast area tonight and Tuesday. The next frontal system will cause rain to increase along the coast Tuesday night, then spread inland for a generally wet day Wednesday. The parent upper trough will keep showers into Thursday, then perhaps some drier and milder weather Friday as high pressure builds across the region. Another system may bring some rain back for next weekend, but details on this remain uncertain. && .SHORT TERM...(Today through Wednesday)...The progressive pattern continues across the North Pacific and into the Pac NW, with a fast, mostly zonal jet stream extending from China to the U.S. West Coast and British Columbia. Only brief and modest changes are expected in this overall pattern for the next 5-7 days at least; mostly in the Wed/Thu time frame. This pattern will likely continue to push frontal systems into the Pac NW every 2-3 days, with the next one likely to affect our region Tue night/Wed. Focusing on what's going on now, showers persist in onshore flow coupled with an upper level trough pushing into the Pac NW. GOES-16 low-level water vapor channel seems to show a decent dry slot approaching the Oregon Coast, which will probably be the back edge of any organized shower activity. The relatively drier air should result in increasing sunbreaks today, but 00z NAM/GFS soundings suggest low-level lapse rates will probably be steep enough to allow shallow showers to persist for much of the day...especially over the higher terrain. ODOT webcams suggest snow levels have lowered below Government Camp (4000'), and the ESRL profiler at Troutdale seems to confirm that snow levels are around 3500' at the moment. Expect snow levels to remain in the 3000 to 3500 ft range today, but relatively strong late March solar energy should allow for wet or slushy roads over most passes by midday today. Still, anyone traveling across the Cascades should be prepared for snow today, especially this morning. We will maintain the Winter Weather Advisory for snow in the Cascades through this afternoon. SNOTEL reports seem to suggest 5-10 inches have already fallen, with another 2-5 inches likely in snow showers today. Warm advection aloft and subsidence associated with higher pressure building into the region will likely bring an end to the showers this evening. A flat upper level ridge will temporarily push the Pacific jet stream north toward Vancouver Island tonight and Tue, allowing for a mostly dry and mild day Tuesday for areas south of a Newport to Portland line. Eugene may push 60 degrees Tuesday depending on how many breaks there are in the high clouds. Given the flatness and positive tilt to the upper ridge, it appears unlikely that complete clearing will occur anywhere in our CWA Tue. SW Washington and the North Coast of Oregon may get clipped with occasional light rain Tue as a shortwave trough races across BC/WA. Models are finally showing decent agreement that an upper trough from the Gulf of Alaska will generate a surface low off the Pac NW coast Tue night, pushing another occluding frontal system onshore late Tue night/Wed. This will cause rain to increase along the coast Tue night, spreading inland late Tue night and Wed. 00z ECMWF/06z NAM suggest this system will have a bit better of an atmospheric river associated with it than the past couple of systems, so rainfall amounts may be heavier...with up to 2.5 inches in the higher terrain and 1-2 inches along the coast. Significant downsloping will probably keep rainfall totals around 0.50 inch for most inland valleys. With much of our coast already in the warm sector, southerly gradients may be sufficient to produce some decent wind with this system Tue night/Wed, especially along the coast. Gusts up to 50 mph will be possible for beaches and headlands, with gusts up to 35 mph possible for the Willamette Valley Wed. Due to the prior ridging, snow levels will likely be well above the passes until the upper trough pushes in behind the front late Wed/Wed night. Weagle .LONG TERM...(Wednesday night through Sunday)...Main challenge in the extended this morning was to put together a forecast disregarding the 00z GFS, which seems to cut off an upper low way too soon off the Pac NW coast during the weekend. Strongly prefer the 00z ECMWF for days 4-7, which remains more progressive through the weekend. The ECMWF shown far more run-to-run consistency than the GFS has demonstrated the last few days; both with our midweek system and the evolution of the next system over the weekend. Model agreement is better through Friday, showing our Wednesday rain tapering to showers Wed night/Thu as a cool upper trough pushes in behind our Wednesday frontal system. The upper trough will cause snow levels to drop briefly below the passes Wed night/Thu, with a few inches of snow likely in the Cascades from the post-frontal showers. As mentioned earlier, leaned heavily on the 00z ECMWF for the forecast Fri-Sun, which still shows decent warming/drying Fri and temps possibly reaching the 60s for the inland valleys. The ECMWF then increases high clouds Fri night, with a weak frontal system bringing a chance for rain Sat and cooler temps with a chance of post-frontal showers Sunday. Weagle && .AVIATION...A weak surface trough approaching the coast will bring enhanced shower activity through mid-morning. Think that there will be a mix of VFR and MVFR conditions during this time. Showers will decrease somewhat through the rest of the day as higher pres builds in aloft and begins to stabilize the atmosphere. Conditions should be predominantly VFR from late morning through tonight, but occasional brief MVFR possible with any heavier showers. KPDX AND APPROACHES...Showers continue through tonight. Heavier showers possible this morning, with a mix of VFR and MVFR. Then expect predominantly VFR the rest of today and tonight. Pyle && .MARINE...A weak surface trough is moving through the coastal waters this morning. There will be occasional gusts of 20 to 25 kt, but they shouldn't be frequent enough to justify a small craft advisory. Then higher pres will build over the waters later today and tonight. A frontal boundary will sag south into the waters on Tue, bringing southerly winds gusting 25 to 30 kt. A low forming along the front will approach from the southwest Tue night into Wed morning. This will strengthen the wind field and may bring some gale force gusts. The low is modeled to move onshore over Vancouver Island Wed afternoon. Then high pres will begin to build over the NE Pac, bringing a period of benign conditions with northerly winds Thu and Fri. There is some uncertainty going into next weekend, but it looks likely that the NE Pac high will persist. Seas are running around 7 to 8 ft this morning. A larger westerly swell will be arriving today. Seas will rise back above 10 ft later this morning, and are expected to reach 12 to 14 ft tonight. Seas will then remain above 10 ft for the next few days. Then should finally drop below 10 ft on Thu and remain below 10 ft into next weekend. Pyle && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Winter Weather Advisory until 4 PM PDT this afternoon for Cascades in Lane County-Northern Oregon Cascades. WA...Winter Weather Advisory until 4 PM PDT this afternoon for South Washington Cascades. PZ...Small Craft Advisory for hazardous seas until 5 PM PDT Tuesday for Coastal Waters from Cape Shoalwater WA to Florence OR out 60 nm. Small Craft Advisory for Rough Columbia River Bar until 7 AM PDT this morning. Small Craft Advisory for Rough Columbia River Bar from 3 PM this afternoon to 8 PM PDT this evening. && $$ Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland This discussion is for Northwest Oregon and Southwest Washington from the Cascade crest to 60 nautical miles offshore. The area is commonly referred to as the forecast area.
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